Sperical subtalar implant

ABSTRACT

One embodiment of the invention includes a sinus tarsi implant with a proximal portion that includes a spherical portion. The spherical portion may include apertures for tissue in-growth. The implant design limits patient discomfort due to pressure points. Other embodiments are disclosed.

This application claims priority to and is a continuation of U.S. patentapplication Ser. No. 13/011,330, filed Jan. 21, 2011 and entitled“SPHERICAL SUBTALAR IMPLANT”, which is a divisional of U.S. patentapplication Ser. No. 13/011,243 filed on Jan. 21, 2011, now U.S. Pat.No. 8,545,572, issued on Oct. 1, 2013 and entitled “SUBTALAR IMPLANT”,the content of which is hereby incorporated by reference.

BACKGROUND

Subtalar Arthroesis implants, also known as sinus tarsi implants, helptreat the hyperpronated foot by stabilizing the subtalar joint. Theimplant may be designed to block forward, downward, and/or medialdisplacement of the talus, thereby allowing normal subtalar joint motionwhile limiting excessive pronation.

Subtalar implants, however, often “back out” or “migrate” from theiroriginal implant locations. To mitigate migration, the thread profile ofsubtalar implants may be aggressive or “deep” to increase the “negativespace” of the thread and provide for deeper coupling with tissue. Anegative thread space includes, for example, the troughs between thethread crests. In other words, the negative thread space includes thespace bordered by the main bodies of two adjacent threads, the core bodyof the device, and a line connecting the crests of the two adjacentthreads. The thread crests are the lateral tips or “crests” of thethreads.

However, to create the thread profile for the deep negative space alathe typically removes significant amounts of material from the threadcrest. For example, with conventional thread forms the thread tapersfrom a wider base to a thinner thread crest. Consequently, machining alarger or deeper negative space requires the removal of additionalthread crest area proportionally. As the amount of thread crest surfacearea decreases the inherent load upon the implant is more focused. Thisfocused load results in higher stress levels for the tissue/implantinterface (e.g., pressure points), which may lead to patient discomfortand implant removal.

Pressure points are not caused only by low surface area thread crests.Pressure points may also arise from the general shape of the proximalportion of subtalar implants. For example, conical shaped implants mayflare out laterally as the proximal end of the implant is approached.However, the conical shape (as well as cylindrical portions of implants)often terminates proximally in a dramatic non-rounded fashion, therebyleaving a sharp or low-surface area edge and a corresponding pressurepoint that may possibly be painful.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the present invention willbecome apparent from the appended claims, the following detaileddescription of one or more example embodiments, and the correspondingfigures, in which:

FIG. 1A includes a side view of one embodiment of the invention.

FIG. 1B includes a section view of one embodiment of the invention.

FIG. 1C includes a perspective view of one embodiment of the invention.

FIG. 1D includes a plan view of one embodiment of the invention.

FIG. 1E includes a side view of one embodiment of the invention.

FIG. 1F includes a plan view of one embodiment of the invention.

FIG. 1G includes portion 123 of FIG. 1F.

FIG. 1H includes a plan view of one embodiment of the invention.

FIG. 2A includes a side view of one embodiment of the invention.

FIG. 2B includes a plan view of one embodiment of the invention.

FIG. 2C includes a plan view of one embodiment of the invention.

FIG. 2D includes a section view of one embodiment of the invention.

FIG. 2E includes a perspective view of one embodiment of the invention.

FIG. 3 includes a view of an embodiment of the invention implanted in afoot.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth.However, it is understood that embodiments of the invention may bepracticed without these specific details. Well-known structures andtechniques have not been shown in detail to avoid obscuring anunderstanding of this description. References to “one embodiment”, “anembodiment”, “example embodiment”, “various embodiments” and the likeindicate the embodiment(s) so described may include particular features,structures, or characteristics, but not every embodiment necessarilyincludes the particular features, structures, or characteristics.Further, some embodiments may have some, all, or none of the featuresdescribed for other embodiments. Also, as used herein “first”, “second”,“third” describe a common object and indicate that different instancesof like objects are being referred to. Such adjectives are not intendedto imply the objects so described must be in a given sequence, eithertemporally, spatially, in ranking, or in any other manner. Also, theterms “coupled” and “connected,” along with their derivatives, may beused. In particular embodiments, “connected” may be used to indicatethat two or more elements are in direct physical or electrical contactwith each other and “coupled” may mean that two or more elementsco-operate or interact with each other, but they may or may not be indirect physical or electrical contact. Also, while similar or samenumbers may be used to designate same or similar parts in differentfigures, doing so does not mean all figures including similar or samenumbers constitute a single or same embodiment.

One embodiment of the invention includes a sinus tarsi implant with aninverted thread profile (e.g., having a thread profile that widenstowards the thread crest) having a lip or lips at the thread crest. Thelips may help hold tissue in-growth and give greater purchase to theimplant. The implant may also include voids that traverse threads. Thevoids may create an open area for tissue (e.g., bone, soft tissue) togrow into. The voids may be relatively small and consequently promotefaster tissue in-growth. Thus, the lips and/or voids limit migration.They do this without creating (or at least limiting) sharp edges orpressure points on thread crests that are typically associated withtechniques (e.g., deep negative thread spaces) used to preventmigration. Therefore, the embodiments of the invention limit migrationwhile also limiting patient discomfort.

FIGS. 1A-B include a subtalar implant 100 that includes a cannulatedmain body having a middle portion connected between proximal end 103 anddistal end 102. Distal end 102 is the leading edge that inserts into asinus. Central hollow shaft 113 extends from proximal end 103 to distalend 102 defining longitudinal axis 124. Thread 101 revolves aboutimplant 100 and includes thread crest 130 located adjacent to negativethread space 107 and negative thread space 108. In one embodiment,thread crest 130 includes lip 105 projecting across a portion ofnegative thread space 107 and lip 106, opposite lip 105, projectingacross a portion of negative thread space 108. In an embodiment thread101 may include beveled edge 104. As used herein, thread crest 130describes a portion of the crest for thread 101, which rotates aboutimplant 100.

In an embodiment, lips 105, 106 are manufactured without use of a lathe.Instead, device 100 may be manufactured using titanium or titanium alloypowder and a 3D printer, such as a Direct Metal Laser Sintering (DMLS)or Electron Beam Machining (EBM) device. In an embodiment, lips orflanges 105, 106 may be thought to form a lateral end of the threadshaped in a “T” formation. This “T” formation may also be thought toinclude elements of an “I” beam wherein the lips correspond to the Ibeam flanges and the main thread body corresponds to the web of the Ibeam. As such, the main thread body (i.e., the web or portion leadinglaterally towards the crest) may resist shear forces while the flangesor lips resist bending moments experienced by the thread. Thus, the Ibeam thread efficiently carries bending and shear loads that may beexperienced by implant 100.

However, embodiments are not limited to this configuration and mayinclude, for example, “L” forms where only 1 lip is included. Also,embodiments do not necessarily require lips at all. Further, the lipsneed not be symmetrical. For example, lip 105 may be larger than lip106. Also, the lips may offset from the main thread body at differentangles. Also, while the lips are generally orthogonal to the main threadbody in FIG. 1B, they need not be and may offset from the main body atnon-orthogonal orientations (e.g., 80 or 100 degrees).

Thus, embodiments include inverted threads. An embodiment includes athread crest that is enlarged and thereby provides a larger surface areafor tissue contact. Upon patient loading, where tissue may contact thethread crest with increased force, the larger surface area of theinverted thread may decrease the stress resultant from the load.Pressure points are also lowered due to the reduced stress at thosepressure points. This may consequently reduce patient pain. Also,inverted threads include lips that may function to retain tissue withinthe negative spaces. This may reduce migration. For example, lips 105and/or 106 protrude out from the thread profile to create an overhangthat may hold soft tissues better than traditional thread profiles.Thus, deep negative spaces are possible (which reduce migration) whileavoiding sharpened thread crests (which reduces pressure points) thatmight normally be a by-product of forming such deep negative spaces.

In an embodiment, proximal end 103 is tapered medially (i.e., towardslongitudinal axis 124) or inwardly from the main or middle body existingbetween ends 102, 103. In an embodiment, distal end 102 may be taperedmedially from the main body. Due to tapering, the total volume ofproximal end 103 may be less than the total volume of distal end 102.Also, when both ends 102, 103 are tapered the maximum diameter 118 (FIG.1D) for the implant 100 is included in the middle portion of the device(and not in the proximal or distal ends 102, 103). The tapering of oneor both ends of implant 100 may result in a capsular shape. With taperedproximal end 103, the resultant reduced trailing edge, which may includesoftened or rounded edges, may reduce a pressure point or points thatinterface patient tissue. The turned or “radiused” edge 129 may come invarying sizes such as, for example and without limitation, a rangeextending generally between 0.1 to 0.3 inches. In various embodiments,there may be between 30, 35, 40, 45, 50, 55, 60, 65, 70 degrees of taperfor either or both of the proximal and distal portions. The tapering(e.g., turned edge 129) may facilitate the implant sitting in the sinustarsi without interference from the lateral prominence on the calcaneusbone.

In an embodiment thread 101 may include aperture 114 directly connectedto negative thread space 107 and negative thread space 108. In anembodiment, aperture 114 directly connects to negative thread space 107at a first location and to negative thread space 108 at a secondlocation, and the first location is separated from the second locationby a distance generally less than 1.5 mm. This relatively short pathhelps prevent migration by fostering tissue linking. For example, afterreceiving the implant a patient may wear a “walking boot” for about twoweeks. During that time tissue may grow into spaces in implant 100. Bydecreasing the length of the void 114, there is a shorter distance fortissue in spaces 107, 108 to traverse void 114 and link together (fullyor partially). To the extent this linking is fostered (fully orpartially) during the time the patient wears a walking boot (therebydecreasing load transmitted to implant 100); doing so may prevent orlessen migration and patient discomfort.

As seen in FIG. 1G, aperture 132 may be tilted. As seen in FIG. 1E,aperture 131 may include central axis 133, passing through aperture 131without contacting walls of aperture 131. Axis 133 is oblique ornon-parallel to longitudinal axis 124. This oblique orientation mayprovide more available surface area in which to locate apertures,thereby increasing the total number of possible apertures in device 100.In an embodiment, a series of apertures 131, 134, 135, 136, locatedadjacent multiple thread crests or thread crest portions 137, 138, 139,may align along axis 133.

As seen in FIG. 1B, in an embodiment thread crest 130 is intercepted byhorizontal axis 127, which is perpendicular to longitudinal axis 124.Aperture 114 is also intercepted by horizontal axis 127. Aperture 114 islocated between thread crest 130 and central hollow bore or shaft 113.Thus, aperture 114 is included in the thread (e.g., main thread body)but is not included in the thread crest 130.

Still concerning FIG. 1B, in an embodiment negative space 140 andnegative space 141 are both intercepted by horizontal axis 126, which isperpendicular to longitudinal axis 124. Also, aperture 115 isintercepted by horizontal axis 126 and not connected to central hollowshaft 113.

As seen in FIG. 1F, in an embodiment an aperture includes a lateral wallat radius 119 and a medial wall at radius 120. The differential ordistance between the lateral and medial walls may be generally withinthe range of about 0.01 to about 0.06 inches. This range may provide awidth that is small enough to promote tissue in-growth. Making thedistance too large may adversely affect the structural integrity ofdevice 100. In various embodiments, the apertures may each generallyinclude dimensions between about 350, 400, 450, 500, 550, 600, 650, 700,750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350,1400, 1450, 1500, 1550, 1600, 1650, 1700 microns per side. In variousembodiments the corresponding horizontal cross-sectional area (i.e.,cross-sectional area taken on a horizontal plane) may generally includedimensions between about 160,000 to 850,000 square microns.

In various embodiments, a single thread (or multiple threads) mayinclude a range of apertures generally including 2, 5, 10, 15, 20, 25,30, 35, 40, 45, 50, 55, 60, 60 apertures or more. Also, embodiments mayinclude threads that make various rotations about device 100. Forexample, in FIG. 1A thread 101 makes three complete revolutions aboutdevice 100. In an embodiment, such as with FIG. 1A, thread 101 has athread crest that is continuous and includes no apertures. However, inother embodiments holes may extend across threads, thereby piercingthread crests (e.g., lips 105, 106). For example, there may be one ormore circular horizontal bores extending from one lateral edge of device100 to the opposite lateral edge. Also, slots may be cut longitudinallyacross threads and thread crests.

Thus, in various embodiments voids help incorporate tissue for stabilityand anti-migration properties. The voids may behave like scaffolding fortissue growth. Additionally, the morphology and aperture size may havesignificant effects on the rate of tissue in-growth. For example, thevoids may include a varied shape similar to an hourglass to provide arange of aperture widths. The range of widths may better promote tissuein-growth. For example, an hourglass shape or other shape may mimic thatof a reticulated open cell structure as used in cell scaffolds andbiologic growth depositions. Use of appropriately sized pores and aninterconnected pore structure may promote induction of soft tissuehealing and repair. Cell migration, proliferation, and attachment may beinfluenced by the high surface area presented by shapes such as thosefound in the hourglass apertures. The irregular crevices, points, andmorphology increase surface area and may contribute to cellar strainthereby increasing proliferation as seen in reticulated foams andscaffolds.

Also, the higher number of smaller voids (as opposed to traditionaldevices with a few vertical slots or horizontally bored voids) providesa high volume for tissue growth but does so using smaller paths forlinking (e.g., the small vertical distance of void 114 that links spaces107, 108), which helps promote faster and more effective tissuein-growth. The small in-growth distance allows for solid,interconnecting chains of tissue to form around struts (e.g., materialexisting directly between two apertures) of the implant thread, securelyanchoring the implant and preventing migration.

FIG. 1D illustrates a “double lead” wherein points 116, 117 connote ordefine two starting points for two different threads (and correspondingthread crests).

Various embodiments provide for a wide range of dimensions. Dimension118 generally includes 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55 inchesand beyond (smaller or larger) that particular range. Dimension 119generally includes 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18,0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25 inches and beyond (smaller orlarger) that particular range. Dimension 120 generally includes 0.06,0.07, 0.08, 0.09, 0.10, 0.11, 0.12, 0.13, 0.14, 0.15, 0.16, 0.17, 0.18,0.19, 0.20, inches and beyond (smaller or larger) that particular range.Dimension 109 generally includes 0.45, 0.50, 0.55, 0.60, 0.65, 0.70,0.75 inches and beyond (smaller or larger) that particular range.Dimension 129 generally includes 0.13, 0.14, 0.15, 0.16, 0.17, 0.18,0.19, 0.20, 0.21, 0.22, 0.23, 0.24, 0.25, 0.26, 0.27, 0.28 inches andbeyond (smaller or larger) that particular range. Dimension 112generally includes 0.030, 0.035, 0.040, 0.045, 0.050, 0.055, 0.060,0.065, 0.070 inches and beyond (smaller or larger) that particularrange.

A method in one embodiment of the invention is now addressed. The methodis suitable for use with the various embodiments included in any ofFIGS. 1A-H but is not so limited. Embodiments of the invention mayinclude a one-piece titanium alloy (e.g., ASTM F-136) implant comprisedof diameters of 7 mm to 12 mm intended for the treatment ofhyperpronation. The subtalar implant may be indicated for use intreating the hyperpronated foot and stabilization of the subtalar joint.It may block the forward, downward, and medial displacement of thetalus, thus allowing normal subtalar joint motion but limiting excessivepronation.

A user (e.g., doctor) may make a 2-3 cm incision on the lateral aspectof the foot over the sinus tarsi along the relaxed skin tension lines.The user may identify the deep facia and bluntly dissect such allowingentrance into the lateral sinus tarsi. The user inserts the guide pininto the sinus tarsi from lateral to medial until tenting is notedanterior and slightly inferior to the medial maleollus. The userintroduces the cannulated probe over the guide pin and into the sinustarsi with a gentle twisting motion to slightly dilate the tarsal canal.The user removes the cannulated probe and leaves the guide pin in place.The user chooses the appropriate trial device based on the size andanatomy of the patient. The user then introduces the selected cannulatedtrial device over the guide pin into the sinus tarsi from lateral tomedial until the leading edge of the trial device is ⅓ to half wayacross the subtalar joint. In one embodiment, the leading edge of thetrial device may not cross the longitudinal bisection of the talus(i.e., approximately at middle of talus where sinus tarsi narrowsconsiderably) and the trailing edge of the implant may be more than 5 mmmedial to the lateral wall of the calcaneous. The appropriate trialdevice size may limit abnormal calcaneal eversion and may allowapproximately 2-4 degrees of subtalar joint eversion.

Once the appropriate size trial device is determined, the user may makenote of the depth measurement on the calibrated section of the trialdevice at the skin line and remove the trial device from the joint whileleaving the guide pin in place. The user may place the equivalent sizeimplant (e.g., implant 100) onto the insertion tool and introduce itover the guide pin and thread it into the joint with a clockwiserotation to the predetermined length noted from the depth measurementdetermined from the trial until clinical correction is noted. The use ofintra-operative imaging in the AP and lateral view may be used to verifythe final placement of the implant. In an embodiment, the leading edgeof the implant may be ⅓ to half way across the subtalar joint and theleading edge of the implant may not cross the longitudinal bisection ofthe talus while the trailing edge may be more than 5 mm medial to thelateral wall of the calcaneous.

Once the final placement of the implant has been achieved, the user mayaccess the range of motion of the subtalar joint. A significantreduction of excess subtalar joint pronation should be appreciated. Theuser then removes the insertion tool and the guide pin, irrigates, andthen closes the deep tissue, fascia, subcutaneous tissue, and skinlayers.

As mentioned above, pressure points are not due solely to sharp or lowsurface area thread crests and deep negative thread spaces. Somepressure points are due to implant shape. For example,cylindrical/conical implants may include a middle conical section with asmaller cylindrical section at the distal end and a larger cylindricalsection at the proximal end. Consequently, the implant may noteffectively match the anatomic loading by the talus and calcaneus. Forexample, in the subtalar joint the calcaneal floor has a slight inclinedue to a bony prominence. Thus, a flared out implant (e.g., a conicalimplant) with no or insufficient proximal tapering may contact the bonyprominence in a manner that creates a painful pressure point.

FIGS. 2A-E, however, include multiple embodiments of the invention withproximal heads that lessen painful pressure points. For example, FIG. 2Aincludes an embodiment where subtalar implant 200 comprises a main bodywith middle portion 202 connected to proximal portion 201 and distalportion 203. In FIG. 2D, central hollow shaft 207 extends from proximalportion 201 to distal portion 203 to define longitudinal axis 225.Proximal portion 201 includes spherical portion 204 having athree-dimensional spherical profile with arcuate edge 208. In anembodiment, middle portion 202 may be non-threaded and distal portion203 may be threaded.

As used herein, “arcuate edge” connotes or describes an arc, arch, orcurved edge. An arcuate edge is found in, for example, ellipses. Anellipse includes a curved line where generally the sum of the distancesfrom two points (foci) to every point on the line is constant. Theposition of the foci determine how “squashed” the ellipse is. A circleis a special case of an ellipse. In an ellipse, if the major and minoraxis are the same length then a circle is rendered, with both foci atthe center. Embodiments herein may include an arcuate edge in an implantportion that is elliptical. However, other embodiments may includearcuate edges that, while curved and non-linear, may not necessarilyconstitute portions of an ellipse, such as a circle. Also, as shown inFIG. 2D, elliptical or spherical portion 204 need not be perfectlyspherical considering, for example, proximal end 201 couples with middleportion 202. Also, spherical portion 204 may include non-arcuate edges,such as proximal-most end 230, which is non-arcuate (e.g., flattened).

In FIG. 2D, an embodiment has spherical portion 204 that includes manyapertures. Aperture 206 is highlighted for ease of description. In anembodiment, aperture 206 does not directly connect to central hollowshaft 207 (however in other embodiments it may do so). In an embodiment,the apertures may connect with each other. For example, in FIG. 2Eapertures 219, 220, 221, 222 may couple to one another via a voidlocated inside the spherical portion. For example, aperture 219 maycouple to aperture 220 via void or channel 231. Aperture 219 may coupleto aperture 222 via void or channel 232. In an embodiment aperture 219may couple to aperture 220 via void or channel 231 and aperture 219 maycouple to aperture 222 via void or channel 232.

As described above regarding, for example, hole 114 of FIG. 1B, thevoids or apertures of FIG. 2D may incorporate tissue for stability andanti-migration properties. The apertures may operate like scaffolding.Additionally, the morphology and aperture size, described below, mayaccelerate tissue in-growth. The voids may include any number of profileshapes including, for example, square, circular, rectangular, and/orhourglass shapes. Hourglass shapes, along with other shapes, may providea range of aperture widths whose diversity of widths promotes tissuegrowth. With a greater number of smaller tissue integration voids, asimilar or greater total volume of in-growth may be achieved (ascompared to traditional designs) while achieving integration at a muchfaster rate. For example, the small distance (e.g., distance 213) fortissue growth may better foster solid, interconnecting chains of tissueforming in shorter periods of time (e.g., during post-operative periodswhere a boot is worn) securely anchoring the implant and preventingmigration.

In an embodiment, spherical portion 204 may include 10, 15, 20, 25, 30,35, 40, 45, 50, 55, 60, 65, 70, 75, 80, 85 or more apertures. Theapertures may be arranged in a series of rings respectively located onhorizontal planes 226, 227 (both orthogonal to longitudinal axis 225)and the like. In an embodiment, apertures (e.g., aperture 206) mayinclude dimensions of about, 350, 400, 450, 500, 550, 600, 650, 700,750, 800, 850, 900, 950, 1000, 1050, 1100, 1150, 1200, 1250, 1300, 1350,1400, 1450, 1500, 1550, 1600, 1650, 1700 microns per side. In anembodiment, apertures (e.g., aperture 206) may include a correspondinghorizontal cross-sectional area of about 160,000 to 850,000 squaremicrons.

In an embodiment shown in FIG. 2A, distal portion 203 includes maximumdiameter 211, which is based on an outer diameter of thread 235. Middleportion 202 includes maximum diameter 236. Spherical proximal portionincludes maximum diameter 212 (FIG. 2C). In an embodiment, maximumdiameter 211 is larger than maximum diameter 236 but smaller thanmaximum diameter 212.

Various embodiments provide for a wide range of dimensions. In anembodiment, diameter 211 generally includes 0.12, 0.14, 0.16, 0.18,0.20, 0.22, 0.24, 0.26, 0.28, 0.30 inches and beyond (smaller or larger)that particular range. Diameter 212 generally includes 0.26, 0.28, 0.30,0.31, 0.33, 0.35, 0.37, 0.39, 0.41, 0.43, 0.45, 0.47, 0.49, 0.51 inchesand beyond (smaller or larger) that particular range. Diameter 236 isgenerally 0.02 to 0.08 inches smaller than diameter 211. Length 209generally includes 0.59, 0.61, 0.63, 0.65, 0.67, 0.69, 0.71, 0.73, 0.75,0.77, 0.79, 0.81, 0.83, 0.85, 0.87, 0.89, 0.91, 0.93 inches and beyond(smaller or larger) that particular range. Length 210 generally includes0.26, 0.28, 0.30, 0.31, 0.33, 0.35, 0.37, 0.39, 0.41, 0.43, 0.45, 0.47,0.49, 0.51 inches and beyond (smaller or larger) that particular range.Distance 237 generally includes 0.02, 0.03, 0.04, 0.05, 0.06 inches andbeyond (smaller or larger) that particular range. Distance 213 generallyincludes 0.02, 0.03, 0.04, 0.05, 0.06 inches and beyond (smaller orlarger) that particular range. Distance 214 generally includes 0.02,0.03, 0.04, 0.05, 0.06 inches and beyond (smaller or larger) thatparticular range. Embodiments may include various materials such assteel, titanium alloy (e.g., ASTM F-136), medical grade polymer, and thelike.

As seen in FIG. 2A, in an embodiment arcuate edge 208 has arc 240extending at least 100° in a longitudinal plane parallel to longitudinalaxis 225. Other embodiments are not so limited and may have arcsextending, for example, 60, 65, 70, 75, 80, 85, 90, 95, 105, 110, 115°and beyond (smaller or larger) that particular range. For example, someembodiments may include no flattened top 230 and may instead extend edge208 across the proximal end and back to middle portion 202, therebyextending, for example, 270, 275, 280, 285, 290, 295, 300, 305, 310,315° and beyond (smaller or larger) that particular range.

As seen in FIGS. 2B and 2C, embodiments of the invention may includearcuate edge 241 that is circular and extends 360° in a horizontal planeorthogonal to longitudinal axis 225.

Embodiments of the invention may combine elements from (for example)FIGS. 1B and 2D. For example, while not shown in a figure, a thread(e.g., thread 205) of implant 200 may include an inverted thread.Specifically, as described more fully above in regards to variousembodiments in FIGS. 1A-H, a thread may be included in distal portion203 and revolve about the distal portion. A first negative thread spacemay be located distal and directly adjacent to the first thread crestportion and a second negative thread space may be located proximal anddirectly adjacent to the first thread crest portion. The first threadcrest portion may include a first lip projecting across a lateralportion of the first negative thread space and a second lip, oppositethe first lip, may project across a lateral portion of the secondnegative thread space. In an embodiment, the thread may include anaperture directly connected to both the first negative thread space andthe second negative thread space. In an embodiment, the first aperturemay be included in the first thread but not in the thread crest.

Various embodiments include methods for implanting devices such asimplant 200 of FIG. 2E. In one embodiment, implant 200 of FIG. 2E isscrewed into a sinus tarsi of a patient. In FIG. 3, distal portion 303of the implant may be proportioned to implant past the longitudinalbi-section 393 of the talus 390 (i.e., approximately at the middle oftalus 390 where the sinus tarsi 394 narrows considerably) and into thedeeper canalis portion 391 of sinus tarsi 394. Spherical proximalportion 304 may abut the entry to deeper canalis 391, thereby providingmotion blocking. The user may directly connect a first arcuate edge,located on an elliptical proximal portion 304 of the implant, to thetalus 390 of the patient at a first contact point. The user may alsodirectly connect a second arcuate edge, also located on an ellipticalproximal portion 304 of the implant, to the calcaneus 392 of the patientat a second contact point.

In an embodiment, the user may locate the proximal-most end of theimplant a distance 395 that is lateral (e.g., 0 to 3 mm) or medial(e.g., 0 to 3 mm) from the lateral edge of the talus 390 upon finalimplantation. Considering the trailing edge of the implant may now havemore contact with bone due to the implant's deep insertion, theelliptical proximal portion (e.g., spherical) may help mitigate pressurepoints associated with contact near the trailing edge of the implant.

During implant and immediately thereafter, the elliptical shape of theproximal portion of the implant (e.g., head or head region) may providea uniform bearing surface at any angle of contact between the talus andcalcaneus to provide uniform block of the joint motion while alsopreventing stress points that may be attributed to hard edges. Thecurved portion may help accommodate certain anatomical features such asthe calcaneus bony prominence on the floor of the sinus tarsi. Incontrast, a conical implant may not provide uniform block of jointmotion due to, for example, anatomical considerations (e.g., calcaneusbony prominence) and/or physician implant technique. For example, if theconical implant is implanted at too steep an angle, a sharp proximaledge of a conical section may place undue pressure on the talus. If theconical implant is implanted at too shallow an angle, a sharp proximaledge of a conical section may put undue pressure on the calcaneous.

Thus, with certain embodiments in a first position the arcuate edge willhave a first incident angle or angle of contact with the calcaneus ofthe patient. But even if the angle of insertion changes (due to userchoice, settling, migration, or the like) and the implant is partiallyrotated about a horizontal axis of the implant or shifts along thelongitudinal axis of the implant (e.g., due to user choice, migration,and the like) the arcuate edge would still maintain the first angle ofcontact with the calcaneus of the patient. This may provide a margin oferror for implantation, migration, and the like.

Also, due to the elliptical shape (e.g., ovular, spheroid) of theproximal portion of the implant, even if the implant settles or changesposition over time, the angle of contact between the talus and theelliptical section and the angle of contact between the calcaneus andthe elliptical section will not necessarily change.

Consequently, the risk for present (at time of implantation) and future(days, weeks, or years after implantation) pressure points isdiminished. This may decrease damage to the bone (which may occur due tothe implant material being harder than bone) and/or patient pain.

A method in one embodiment of the invention is now addressed. The methodis suitable for use with various embodiments in FIGS. 2A-E but is not solimited. A user may make a 2-3 cm incision on the lateral aspect of thefoot over the sinus tarsi along the relaxed skin tension lines. The usermay identify the deep facia and bluntly dissect such allowing entranceinto the lateral sinus tarsi. The user inserts the guide pin into thesinus tarsi from anterior lateral to posterior medial until tenting isnoted slightly posterior to the medial maleollus. The user chooses anappropriate trial device based on the size and anatomy of the patient.The user may introduce the selected cannulated trial device over theguide pin into the sinus tarsi and canalis tarsi from anterior lateralto posterior medial until the trial will not advance anymore. Theappropriate trial size should limit abnormal calcaneal eversion and mayallow approximately 2-4 degrees of subtalar joint eversion.

The user then places the equivalent size implant onto the insertion tooland introduces it over the guide pin and threads it into the joint witha clockwise rotation. Once the implant has been advanced 3-4 full turnsinto the canalis tarsi, the user removes the guide pin and fully seatsthe implant until it does not advance any further and final placementmatches the predetermined length noted from the depth measurementdetermined from the trial until clinical correction is noted. The use ofintra-operative imaging in the AP and lateral view may verify the finalplacement of the implant. In an embodiment, the trailing edge of theimplant may sit +/−2 mm from the neck of the talus.

Once the final placement of the implant has been achieved, the user mayassess the range of motion of the subtalar joint. A significantreduction of excess subtalar joint pronation should be appreciated. Theuser may then irrigate and close the deep tissue, fascia, subcutaneoustissue, and skin layers.

While embodiments of the invention have been mentioned in terms of asubtalar implant, claim scope is not necessarily so limited. Embodimentsare suitable for other implantation sites, such as other joints, bones(of the foot and elsewhere in the body), humans and other animals, andthe like. Several embodiments are described as being cannulated butother embodiments may be uncannulated with no central hollow shaft.Embodiments may include various materials such as steel, titanium alloy(e.g., ASTM F-136), medical grade polymer (e.g., high molecular weightpolyethylene, PEEK, PEKK, PMMA, PTFE), and the like. Also, while thepresent invention has been described with respect to a limited number ofembodiments, those skilled in the art will appreciate numerousmodifications and variations therefrom. It is intended that the appendedclaims cover all such modifications and variations as fall within thetrue spirit and scope of this present invention.

What is claimed is:
 1. A subtalar implant comprising: a central hollowshaft defining a longitudinal axis and forming a tunnel; and a generallyspherical portion having a three-dimensional spherical profile with anarcuate edge; wherein the spherical portion includes apertures that (a)do not directly connect to the central hollow shaft, but (b) do couplewith each other via a void, which is not coterminous with the centralhollow shaft, located inside the spherical portion.
 2. The implant ofclaim 1, wherein a middle portion of the implant is non-threaded and adistal portion of the implant is threaded.
 3. The implant of claim 1,wherein the apertures includes more than 15 apertures arranged in aseries of rings.
 4. The implant of claim 1, wherein each of theapertures includes a horizontal cross-sectional area between about160,000 and 850,000 square microns.
 5. The implant of claim 1, wherein(a) a distal portion of the implant includes a first maximum diameter,which is based on an outer diameter of a thread included in the distalportion, a middle portion of the implant includes a second maximumdiameter, and the spherical portion includes a third maximum diameter,(b) the first maximum diameter is larger than the second maximumdiameter but smaller than the third maximum diameter, and (c) the distalportion is proportioned to implant within the deeper canalis portion ofthe sinus tarsi of the patient.
 6. The implant of claim 1, wherein thearcuate edge has an arc extending at least 100° in a longitudinal planeparallel to the longitudinal axis.
 7. The implant of claim 6, whereinthe spherical portion includes an additional arcuate edge that iscircular and extends 360° in a horizontal plane orthogonal to thelongitudinal axis.
 8. The implant of claim 7, wherein the sphericalportion includes a proximal-most end that is non-arcuate and coterminouswith a proximal-most end of the implant.
 9. The implant of claim 1comprising: a first thread included in a distal portion of the implantand revolving about the distal portion, the first thread including afirst thread crest portion; a first negative thread space located distaland directly adjacent to the first thread crest portion; a secondnegative thread space located proximal and directly adjacent to thefirst thread crest portion; wherein the first thread crest portionincludes a first lip projecting across a lateral portion of the firstnegative thread space and a second lip, opposite the first lip,projecting across a lateral portion of the second negative thread space.10. The implant of claim 9, wherein the first thread includes a firstaperture directly connected to both the first negative thread space andthe second negative thread space.
 11. The implant of claim 10, whereinthe first aperture is included in the first thread but is not includedin the first thread crest portion.
 12. The implant of claim 1, whereinthe spherical portion includes an additional arcuate edge that iscircular and extends 360° in a horizontal plane orthogonal to thelongitudinal axis.
 13. The implant of claim 1, wherein (a) in a firstposition the spherical portion is configured to contact the calcaneus ofthe patient at a first location at a first angle of contact, and (b) ina second position, where the implant is rotated less than 360° about ahorizontal axis orthogonal to the longitudinal axis, the sphericalportion is configured to contact the calcaneus at a second location yetstill maintain the first angle of contact with the calcaneus based onthe shape of the arcuate edge.
 14. The implant of claim 6, wherein thearc has a midpoint located at the maximum diameter of a proximal portionof the implant and half of the arc is proximal to the midpoint and theother half of the arc is distal to the midpoint.